1. Field of the Invention
This invention relates to temperature sensitive circuits that provide a signal when the temperature exceeds predetermined limits, and more particularly to such circuits that have a hysteresis capability which holds the signal on until the temperature has returned into the permissible range by a hysteresis differential.
2. Description of the Prior Art
It would be desirable to provide a temperature sensitive circuit that generates a warning signal when the device is either above or below a specific temperature range. Such a device would also preferably have a hysteresis capability such that the warning signal is not discontinued immediately when the temperature returns to the permissible range, but rather stays on until the temperature has continued into that range by a hysteresis differential. This prevents a jittering of the circuit's output when the temperature is hovering at the limit of the permissible range and intermittently slips into and out of that range, and also provides a longer warning signal when an over- or under-temperature situation has occurred.
Comparator circuits are presently available with a hysteresis capability that can be either designed into the unit, or selected by the user through the addition of external feedback circuitry. A comparator provides a logic output that indicates the amplitude relationship between two analog signal inputs; an output signal is produced when the differential between the two input signals exceeds a predetermined amount. The COMP-08 high-speed comparator by Analog Devices, Inc., the assignee of the present invention, is an example of such a device. Its hysteresis capability is useful in providing sharp output transitions even when the slew rates of its inputs are relatively slow, and in reducing the likelihood of invalid output transitions due to noise.
A schematic diagram of the COMP-08 hysteresis circuitry is provided in FIG. 1. The comparator itself is designated by numeral 2, with a variable input applied at input terminal 4 and a reference input applied through resistor Ra at input terminal 6. The comparator output is delivered to an output terminal 8, while a complementary output is connected back to its reference input through a feedback resistor Rb. Equal value pulldown resistors RL1 and RL2 are connected respectively from the comparator's output and its complementary output to a termination voltage V.sub.T terminal 10. With Ra equal to 10 ohms and Rb equal to 4.7 kohms, switching points at input voltages of -1.1 mV and -3.9 mV are typically obtained. The hysteresis trip points may be offset by connecting Ra to a reference voltage other than ground.
While the described comparator is suitable for its intended purpose, it merely compares a variable input voltage to a reference, rather than signaling when the temperature has exceeded either the upper or lower limits of a desired temperature range. Even if the variable input signal represented temperature, the circuit would produce an output signal only when the input exceeded one end of a temperature range, not both ends. The hysteresis circuitry is likewise applicable to only one end of a temperature range.
While an aggregation of comparators and respective hysteresis circuits might be envisioned that collectively produce warning signals when either end of a given temperature range is exceeded, it would be very useful to be able to vary the amount of hysteresis at will. However, the addition of a new function such as hysteresis settability typically requires the dedication of an output pin to that function. Since the number of available pins on a given device is often quite limited and all of the pins might already be required for some other purpose, as a practical matter the device may be unable to accommodate a user-controlled hysteresis capability.